The present invention integrates a varactor diode which has a very large tuning ratio (i.e. a wide-ratio diode) in a monolithic microwave integrated circuit (MMIC), to provide a wide-band microwave voltagecontrolled oscillator (VCO).
Conventional varactor diodes, particularly those with large tuning ratios (hyperabrupt diodes) require highly conductive substrate material and relatively thick epitaxial layers (greater than one micron). These material requirements are not compatible with the requirements of GaAs FET-monolithic microwave integrated circuits (MMICS) which require a thin (less than one-half micron) uniformly doped active layer on a semi-insulating substrate. To integrate the conventional hyperabrupt diode on a semi-insulating substrate requires a very complicated selective epitaxial deposition, wherein certain areas of the substrate surface receive one epitaxial layer, and other areas receive a different epitaxial layer. The materials required to implement a varactor in an MMIC should be the same as or similar to those for an FET, so that varactors can easily be integrated in, e.g., monolithic microwave voltage controlled oscillators.
Thus, it is an object of the present invention to provide a monolithic microwave integrated circuit incorporating a wide-ratio varactor in a thin uniformally doped active layer above a semi-insulating substrate.
R. VanTuyl, "A Monolithic GaAs FET RF Signal Generation Chip", ISSCC-80 Digest 118 (which is hereby incorporated by reference) discloses a gallium arsenide varactor diode in an MMIC which is integrated in a thin epitaxial layer on a semi-insulating substrate. The VanTuyl device does not, however, provide very wide capacitance tuning characteristics. A wide capacitance range (of a decade or more) is essential for many microwave applications. In addition, the VanTuyl device is designed for operation only at lower microwave frequencies (of at most 4 GHz).
The frequency tuning range of a varactor-based VCO is much narrower than the capacitance range of the tuning varactor, due to the inherent and parasitic reactance characteristics of FETs and other components of the VCO. In particular, an extremely wide-range varactor (having a capacitance ratio of a decade or more) is needed if the frequency range of the VCO is to remotely approach one octave.
Thus, it is an object of the present invention to provide a VCO having a tuning range of 1.5 to 1 or larger at microwave frequencies. It is a further object of the present invention to provide a VCO having a tuning range of 1.3 to 1 or better at microwave frequencies above 5 GHz.
It is a further object of the present invention to provide a microwave VCO having a tuning range of an octave or more.
It is a further object of the present invention to provide a monolithic microwave VCO having a tuning range of 1.5 to 1 or larger.
A major difficulty which arises in microwave VCOs having such a large frequency range is maintaining the correct impedance match to achieve the maximum obtainable bandwidth. Mismatch can easily become such as to gravely impair performance.
However, in a monolithic microwave integrated circuits even trimming (to achieve impedance match at one particular frequency) is difficult, and optimal matching over a wide range of frequencies is presently impossible.
Thus, it is a further object of the present invention to provide means for maintaining impedance matching of a monolithic microwave wide-band VCO over a very large frequency range.
At the present, YIG devices provide the only practical means of wide tuning at higher microwave frequencies, e.g., 10 GHz. However, not only do such YIG structures require very complex magnetic-field-controlled tuning structures, but also the capacitance (or frequency) change permitted by YIG devices is very slow, on the order of a millisecond or more.
Thus, it is a further object of the present invention to provide a microwave reactance which can be tuned over an extremely wide range with great rapidity. In particular, it is an object of the present invention to provide a microwave reactance which can be tuned over a wide range with great rapidity at frequencies in excess of 5 GHz.
Some examples of prior art integrated varactors include: U.S. Pat. Nos. 3,396,312 to Vendelin, 3,559,005 to Vandelin et al, and 3,636,420 to Vandelin et al.